Process for purifying a raw gas via membrane separation with membrane gas supply temperature monitoring

Abstract

The present invention relates to a process for purifying a raw gas via membrane separation, the process using n membrane separation stages, n being greater than or equal to 1, the gas to be treated consisting of a mixture of at least two components A and B, the process being characterised in that it comprises the following steps: establishing a nominal flow rate and / or a nominal composition of the incoming raw gas to be treated; for each separation stage i, determining at least one nominal operating characteristic variable from among the ratio RSCi 0, the loading rate of component A or B at stage i, respectively denoted by TCA i 0 and TCB i 0, the overall loading rate of component A or B at stage i, respectively denoted by TCGA i 0 and TCGB i 0 and the specific consumption CSP0, wherein, when the flow rate and / or the composition of the incoming raw gas changes during the course of the process, the permeability of at least one membrane in at least one stage i is adjusted by modifying the supply temperature of the corresponding separation stage so that at all times, and independently for each stage i, at least one from among the ratio RSCi or the loading rate TCA i or TCB i or the overall loading rate TCGA i or TCGB i or the CSP, measured and / or calculated in real time at each separation stage, is maintained close to its corresponding reference value, RSCi 0 or TCA i 0 or TCB i 0 or TCGA i 0 or TCGB i 0 or CSP0.

Description

Process for purifying a raw gas by membrane separation with control of the gas supply temperature to the membranes

[0001] The present invention relates to a method for purifying a raw gas by membrane separation with control of the gas supply temperature to the membranes. As an example of gas to be purified, mention may be made of biogas from which it is desired to obtain a stream rich in carbon dioxide (CO2) and a stream rich in methane (CH4).

[0002] An example of a process for purifying a raw gas is described by European patent EP 2 588 217 B. In this process, three membrane separation stages are used, the raw gas being a mixture of at least two components A and B, A being the component with the highest permeance.

[0003] This process, which aims to separate the raw gas stream into a stream enriched in A and a stream enriched in B, is illustrated by the diagram of the.

[0004] As can be seen in this diagram: the raw gas stream 1 is compressed in a compressor C1 to obtain a compressed gas stream 2 which is passed into a first membrane separation stage E1 to obtain: a retentate stream 3; and a permeate stream 4; the retentate stream 3 is passed into a second membrane separation stage E2 to obtain: a retentate stream 5, which constitutes the desired B-enriched stream; and

[0005] a permeate stream 6; the permeate stream 4 is passed into a third membrane separation stage E3 to obtain: a retentate stream 7; and a permeate stream 8, which constitutes the desired A-enriched stream.

[0006] A pressure reducing valve V1 is placed in the path of stream 7. The permeate stream 6 and the retentate stream 7 are recycled into stream 1 before introduction into compressor C1.

[0007] When operating conditions deviate from a nominal operating point defined from the operating conditions used for a specific flow rate and / or a specific composition of raw gas to be purified, the performance of the purification process will be affected by this deviation from the nominal operating point. It would therefore be desirable to have a simple means of controlling the process in order to be able to modify the operating conditions so as to regain performance identical or close to that of the nominal operating point when the flow rate and / or composition conditions of the raw gas entering the system change.

[0008] To this end, the present invention relates to a method for purifying a raw gas by membrane separation, said method using n membrane separation stages, n being greater than or equal to 1, the or each membrane separation stage, denoted i with 1 ≤ i ≤ n, being constituted by:at least one separation membrane whose permeability depends on the temperature;a gas feed inlet of said stage i through which:for i=1, said raw gas to be treated is introduced, where appropriate in combination when n=1 with a permeate recycling stream and / or a retentate recycling stream from the single stage and when n>1 with a permeate recycling stream and / or a retentate recycling stream, said recycling streams coming from another stage; andfor i>1, the permeate or the retentate from one of the other stages is introduced,a retentate outlet; anda permeate outlet,

[0009] said gas to be treated consisting of a mixture of at least two constituents A and B, the permeability of the membranes for constituent A being greater than that of constituent B,

[0010] characterized by the fact that it comprises the following steps: a nominal flow rate and / or a nominal composition of the incoming raw gas to be treated is established, representing the nominal operating point; for each separation stage i, at least one nominal operating characteristic quantity chosen from: the RSC ratio is determined i 0, defined as the SC ratio A i 0 / SC B i 0 or SC B i 0 / SC A i 0,

[0011] SC A i 0 and SC B i 0being each defined as the ratio of the partial flow rate of the constituent A or B respectively in the permeate stream leaving stage i, noted respectively Pe Ai 0 and Pe B i 0, on the partial flow of the same constituent in the supply of the same stage i, noted respectively F A i 0 and F B i 0; the charge rate of component A or B at stage i, noted respectively TC A i 0 and TC B i 0, defined as the ratio F A i 0 / P A i 0 or F B i 0 / P B i 0,

[0012] F A i 0 and F B i 0 being the partial flow rate of component A or B in the supply of stage i and P A i 0 or P B i 0the permeability of the membranes at the temperature of the nominal operating point for component A or B on stage i;the overall loading rate of component A or B on stage i, noted respectively TCG A i 0 and TCG B i0, defined as the ratio of the flow rate of component A or B in the nominal flow rate to the permeability of the membranes at the temperature of the nominal operating point for the component A or B respectively on stage i, i.e. P A i 0 or P B i0; and in the case where operation is carried out with several stages, where identical membranes are used on all stages i and the same feed temperature is established on all stages i for the nominal flow rate and / or the nominal composition of the incoming raw gas to be treated, the specific consumption CSP0, defined as the energy consumption of the process divided by the nominal flow rate of an incoming raw gas to be treated or of at least one stream leaving the process under the established nominal conditions;when the flow rate and / or composition of the incoming raw gas, measured before any possible mixing with at least one recycling stream, changes during the course of the process, the permeability of at least one membrane in at least one stage i is adjusted and thereby the quantity of constituents A and B passing through the membrane(s) concerned, by adjusting the temperature of the feed to the corresponding separation stage so that at any time, and independently for each stage i, at least one of the RSC ratio; i or the TC charge rate A i or TC B i or the overall TCG charge rate A i or TCG B i or the CSP, measured and / or calculated in real time on each separation stage, approaches its corresponding reference value, RSC i 0 or TC A i 0 or TC B i 0 or TCG A i 0 or TCG B i0 or CSP0.

[0013] The permeability or permeance of a membrane for a constituent is an intrinsic property of the material constituting the membrane wall to allow said constituent to pass through it. This permeability can be expressed in the form of a molar flow rate per unit area and per unit pressure, i.e. in mol / s / m 2 / Pa. The dependence of membrane permeability on temperature can, for example, follow an Arrhenius-type relationship.

[0014] Selectivity is the ratio of the permeability of a first pure constituent to the permeability of a second pure constituent. Selectivity also varies with temperature.

[0015] In a particular embodiment according to which one operates with several stages, one uses identical membranes on all stages i and one establishes the same supply temperature on all stages i for the nominal flow rate and / or the nominal composition of the incoming raw gas to be treated, one chooses as the nominal characteristic quantity the overall loading rate TCG A i 0 or TCG B i 0, and in the case of a change in the flow rate and / or composition of the incoming raw gas, the feed temperature is adjusted in all stages using a single regulator associated with the overall load rate at any stage.

[0016] At any time and in real time and for each stage i, we can determine the permeability of the membranes for the component A or B, or P A i (t) or P B i(t) by measurement and / or calculation of the pressures, temperatures, flow rates and compositions of the incoming and outgoing streams, and by numerical resolution.

[0017] Knowledge of the pressures, temperatures, flow rates and compositions of the incoming and outgoing streams makes it possible to establish material balances on a membrane. From physical models of these membranes and conventional numerical resolution methods, the person skilled in the art can access the permeability values.

[0018] For example, it is possible to use the physical models and numerical resolution methods described by Michael Binns et al. in Strategies for the simulation of multi-component hollow fibre multi-stage membrane gas separation systems, Journal of Membrane Science, 497, (2016), 458–471.

[0019] The separation membranes may be polymer membranes selected from polysulfone membranes and polyimide membranes.

[0020] The feed temperature of each stage can be adjusted using a heat exchanger. It is also possible to use any temperature adjustment means known to the person skilled in the art.

[0021] In a particular embodiment, component A of the raw gas is carbon dioxide, and component B is methane. Description of Figures

[0022] Lais a schematic representation of a typical process serving as a basis for the process according to the invention;Lais a schematic representation of the process according to a first embodiment with 3 cascaded control loops, regulating the RSC ratios;Lais a schematic representation of the process according to a second embodiment with 3 cascaded control loops, regulating the TC load rates;Lais a schematic representation of the process according to a third embodiment with 3 cascaded control loops, regulating the overall TCG load rates;Lais a schematic representation of the process according to a fourth embodiment with 1 cascaded control loop, regulating the overall TCG load rate; andLais a schematic representation of the process according to a fifth embodiment with 1 cascaded control loop, regulating the CSP consumption.

[0023] This is a resumption of the, with the installation of: a temperature regulation device H1 on the path of the current 2 to modify the temperature of the feed 2' of the first stage E1; a temperature regulation device H2 on the path of the retentate 3, to modify the temperature of the feed 3' of the second stage E2; and a temperature regulation device H3 on the path of the permeate 4, to modify the temperature of the feed 4' of the third stage E3.

[0024] the same reference numbers and letters as on the having been retained to designate similar pipes and devices / apparatus.

[0025] On the, the following regulation organs appear:T_F1, temperature sensor whose role is to measure the temperature of the 2' supply of the first stage E1;T_F2, temperature sensor whose role is to measure the temperature of the 3' supply of the second stage E2;T_F3, temperature sensor whose role is to measure the temperature of the 4' supply of the third stage E3;PT, pressure sensor placed on the retentate stream 7 of E3, upstream of V1; andThe regulation organs noted by blocks:SP_INT: Internal adjustment point (setpoint) of the master regulatorSP_EXT: External adjustment point (setpoint) of the slave regulator corresponding to the output of the master regulatorSP_MAX: Maximum adjustment pointMESURE: Measurement entering the regulatorOUT: Output of the regulatorOUT_MAX: Maximum output of the regulatorOUT_MIN: Minimum output of the regulator

[0026] and bearing the reference letters OR1 to OR8.

[0027] The OR1 and OR2 components (2 cascade regulators) regulate the purification efficiency of the process. The purification efficiency is defined as the flow rate of component B recovered from the retentate 5 divided by the flow rate of component B entering the raw gas 1.

[0028] The desired setpoint of the purification efficiency is entered as SP_INT value of the OR1 device. It also receives the measured value of the flow rate of component B in retentate 5 divided by the flow rate of component B entering raw gas 1 as MEASUREMENT value.

[0029] The OR2 organ receives as MEASUREMENT value the pressure of the retentate 7 measured by the pressure sensor PT.

[0030] By comparing the desired setpoint value and the measured value of the purification efficiency, the OR1 component will send as output value OUT to the OR2 component the pressure value to be reached for the retentate 7, the OR2 component receiving it as value SP_EXT.

[0031] The OR2 organ will then control the pressure of the retentate 7 by sending the output value OUT to the control valve V1 in order to maintain the value of the purification efficiency.

[0032] The OR3 and OR4 organs constitute a cascade regulation of two regulators whose function is to control the RSC ratio 1 of the first membrane separation stage E1 by acting on the temperature of the feed 2' of the first stage by means of the temperature regulation device H1.

[0033] The desired setpoint value of the RSC ratio 1 0 under nominal conditions is entered as SP_INT value of the OR3 organ and the OR3 organ receives as MEASUREMENT value the value of the RSC ratio 1 calculated in real time. The output of the OR3 device is a working temperature setpoint sent in SP_EXT to the OR4 device.

[0034] The OR4 device compares the SP_EXT setpoint from OR3 to the temperature MEASUREMENT value of the supply 2' of the first stage E1 measured by the temperature sensor T_F1 and acts on the temperature control device H1 so that it modifies the temperature of the stream 2 leaving the compressor C1 so that the supply 2' is at the right temperature so that the RSC value 1 approaches the RSC value 1 0.

[0035] The OR5 and OR6 organs constitute a cascade regulation of two regulators whose function is to control the RSC ratio 2 of the second membrane separation stage E2 by acting on the temperature of the feed 3' of the second stage by means of the temperature regulation device H2.

[0036] The desired setpoint value of the RSC ratio 20 in nominal conditions is entered as SP_INT value of the OR5 organ and the OR5 organ receives as MEASUREMENT value the value of the RSC ratio 2 calculated in real time. The output of the OR5 device is a working temperature setpoint sent in SP_EXT to the OR6 device.

[0037] The OR6 device compares the SP_EXT setpoint from OR5 to the temperature MEASUREMENT value of the 3' feed of the second stage E2 measured by the temperature sensor T_F2 and acts on the H2 temperature control device so that it modifies the temperature of the retentate 3 of the first stage E1 so that the 3' feed is at the right temperature so that the RSC value 2 approaches the RSC value 2 0.

[0038] The OR7 and OR8 organs constitute a cascade regulation of two regulators whose function is to control the RSC ratio 3of the third membrane separation stage E3 by acting on the temperature of the feed 4' of the third stage by means of the temperature control device H3.

[0039] The desired setpoint value of the RSC ratio 3 0 in nominal conditions is entered as SP_INT value of the OR7 organ and the OR7 organ receives as MEASUREMENT value the value of the RSC ratio 3 calculated in real time. The output of the OR5 device is a working temperature setpoint sent in SP_EXT to the OR8 device.

[0040] The OR8 device compares the SP_EXT setpoint from OR7 to the temperature MEASUREMENT value of the feed 4' of the third stage E3 measured by the temperature sensor T_F3 and acts on the temperature control device H3 so that it modifies the temperature of the permeate 4 of the first stage E1 so that the feed 4' is at the right temperature so that the RSC value 3 approaches the RSC value3 0.

[0041] This is a resumption of the, with the installation of regulatory bodies OR9 to OR14 in place of regulatory bodies OR3 to OR8, the bodies OR1 and OR2 being identical.

[0042] The OR9 and OR10 organs constitute a cascade regulation of two regulators whose function is to control the TC ratio A 1 or TC B 1 of the first membrane separation stage E1 by acting on the temperature of the feed 2' of the first stage by means of the temperature regulation device H1.

[0043] The desired setpoint value of the TC ratio A 1 0 or TC B 1 0 under nominal conditions is entered as SP_INT value of the OR9 device and the OR9 device receives as MEASUREMENT value the value of the TC ratio A 1 or TC B 1calculated in real time. The output of the OR7 device is a working temperature setpoint sent in SP_EXT to the OR10 device.

[0044] The OR10 device compares the SP_EXT setpoint from OR9 to the temperature MEASUREMENT value of the supply 2' of the first stage E1 measured by the temperature sensor T_F1 and acts on the temperature control device H1 so that it modifies the temperature of the stream 2 leaving the compressor C1 so that the supply 2' is at the right temperature so that the TC value A 1 or TC B 1 approaches the TC value A 1 0 or TC B 1 0correspondent.

[0045] The OR11 and OR12 organs constitute a cascade regulation of two regulators whose function is to control the TC ratio A 2 or TC B 2of the second membrane separation stage E2 by acting on the temperature of the feed 3' of the second stage by means of the temperature regulation device H2.

[0046] The desired setpoint value of the TC ratio A 2 0 or TC B 2 0 under nominal conditions is entered as SP_INT value of the OR11 device and the OR11 device receives as MEASUREMENT value the value of the TC ratio A 2 or TC B 2 calculated in real time. The output of the OR11 device is a working temperature setpoint sent in SP_EXT to the OR12 device.

[0047] The OR12 device compares the SP_EXT setpoint from OR11 to the temperature MEASUREMENT value of the feed 3' of the second stage E2 measured by the temperature sensor T_F2 and acts on the temperature control device H2 so that it modifies the temperature of the retentate 3 of the first stage E1 so that the feed 3' is at the right temperature so that the TC value A 2 or TC B 2 approaches the TC value A 2 0 or TC B 2 0correspondent.

[0048] The OR13 and OR14 organs constitute a cascade regulation of two regulators whose function is to control the TC ratio A 3 or TC B 3 of the third membrane separation stage E3 by acting on the temperature of the feed 4' of the third stage by means of the temperature control device H3.

[0049] The desired setpoint value of the TC ratio A 3 0 or TC B 3 0 under nominal conditions is entered as SP_INT value of the OR13 device and the OR13 device receives as MEASUREMENT value the value of the TC ratio A 3 or TC B 3 calculated in real time. The output of the OR13 device is a working temperature setpoint sent in SP_EXT to the OR14 device.

[0050] The OR14 device compares the SP_EXT setpoint from OR13 to the temperature MEASUREMENT value of the feed 4' of the third stage E3 measured by the temperature sensor T_F3 and acts on the temperature control device H3 so that it modifies the temperature of the permeate 4 of the first stage E1 so that the feed 4' is at the right temperature so that the TC value A 3 or TC B 3 approaches the TC value A 3 0 or TC B 30correspondent.

[0051] This is a resumption of the, with the installation of the regulatory bodies OR15 to OR20 in place of the regulatory bodies OR9 to OR14, the bodies OR1 and OR2 being identical.

[0052] The OR15 and OR16 organs constitute a cascade regulation of two regulators whose function is to control the TCG ratio A 1 or TCG B 1 of the first membrane separation stage E1 by acting on the temperature of the feed 2' of the first stage by means of the temperature regulation device H1.

[0053] The desired setpoint value of the TCG ratio A 1 0 or TCG B 1 0 under nominal conditions is entered as SP_INT value of the OR15 organ and the OR15 organ receives as MEASUREMENT value the value of the TCG ratio A 1 or TCG B 1calculated in real time. The output of the OR15 device is a working temperature setpoint sent in SP_EXT to the OR16 device.

[0054] The OR16 device compares the SP_EXT setpoint from OR15 to the temperature MEASUREMENT value of the supply 2' of the first stage E1 measured by the temperature sensor T_F1 and acts on the temperature control device H1 so that it modifies the temperature of the stream 2 leaving the compressor C1 so that the supply 2' is at the right temperature so that the TCG value A 1 or TCG B 1 approaches the TCG value A 1 0 or TCG B 1 0correspondent.

[0055] The OR17 and OR18 organs constitute a cascade regulation of two regulators whose function is to control the TCG ratio A 2 or TCG B 2of the second membrane separation stage E2 by acting on the temperature of the feed 3' of the second stage by means of the temperature regulation device H2.

[0056] The desired setpoint value of the TCG ratio A 2 0 or TCG B 2 0 under nominal conditions is entered as SP_INT value of the OR17 device and the OR17 device receives as MEASUREMENT value the value of the TCG ratio A 2 or TCG B 2 calculated in real time. The output of the OR17 device is a working temperature setpoint sent in SP_EXT to the OR18 device.

[0057] The OR18 device compares the SP_EXT setpoint from OR17 to the temperature MEASUREMENT value of the 3' feed of the second stage E2 measured by the temperature sensor T_F2 and acts on the H2 temperature control device so that it modifies the temperature of the retentate 3 of the first stage E1 so that the 3' feed is at the correct temperature so that the TCG value A 2 or TCG B 2 approaches the TCG value A 2 0 or TCG B 2 0correspondent.

[0058] The OR19 and OR20 organs constitute a cascade regulation of two regulators whose function is to control the TCG ratio A 3 or TCG B 3 of the third membrane separation stage E3 by acting on the temperature of the feed 4' of the third stage by means of the temperature control device H3.

[0059] The desired setpoint value of the TCG ratio A 3 0 or TCG B 3 0 under nominal conditions is entered as SP_INT value of the OR19 organ and the OR19 organ receives as MEASUREMENT value the value of the TCG ratio A 3 or TCG B 3 calculated in real time. The output of the OR19 device is a working temperature setpoint sent in SP_EXT to the OR20 device.

[0060] The OR20 device compares the SP_EXT setpoint from OR19 to the temperature MEASUREMENT value of the feed 4' of the third stage E3 measured by the temperature sensor T_F3 and acts on the temperature control device H3 so that it modifies the temperature of the permeate 4 of the first stage E1 so that the feed 4' is at the right temperature so that the TCG value A 3 or TCG B 3 approaches the TCG value A 3 0 or TCG B 30correspondent.

[0061] This is a resumption of the, with the installation of regulatory bodies OR21 to OR24 in place of regulatory bodies OR15 to OR20, bodies OR1 and OR2 being identical.

[0062] In this case, the membranes of separation stages E1, E2 and E3 are identical. Under nominal conditions, the temperatures of feeds 2', 3' and 4' are identical. The behavior of all stages is similar.

[0063] Only one master regulator, regulator OR21, is used, in combination with slave regulators OR22 to OR24.

[0064] The OR21 organ and the OR22, OR23 and OR24 organs constitute a cascade regulation whose function is to control the TCG ratio A 1 or TCG B 1of the first membrane separation stage E1 and to act on the temperatures of the feeds 2', 3' and 4' by means, respectively, of the temperature control devices H1, H2 and H3.

[0065] The desired setpoint value of the TCG ratio A 1 0 or TCG B 1 0 under nominal conditions is entered as SP_INT value of the OR21 organ and the OR21 organ receives as MEASUREMENT value the value of the TCG ratio A 1 or TCG B 1 calculated in real time. The output of the OR21 device is a working temperature setpoint sent in SP_EXT to each of the OR22, OR23 and OR24 devices.

[0066] The devices OR22, OR23 and OR24 compare the SP_EXT setpoint from OR21 with the temperature MEASUREMENT value of the power supply 2' of the first stage E1 measured by the temperature sensor T_F1, of the power supply 3' of the second stage E2 measured by the temperature sensor T_F2 and of the power supply 4' of the third stage E3 measured by the temperature sensor T_F3. The devices OR22, OR23 and OR24 then act on the temperature control devices H1, H2 and H3 respectively so that they modify the temperature of the streams 2, 3 and 4 respectively so that the power supplies 2', 3 / and 4' respectively are at the right temperature so that the TCG value A 1 or TCG B 1 approaches the TCG value A 1 0 or TCG B 1 0correspondent.

[0067] This is a resumption of the, with the establishment of regulatory bodies OR25 and OR28 in place of regulatory bodies OR21 to OR24.

[0068] In this case too, the membranes of separation stages E1, E2 and E3 are identical. Under nominal conditions, the temperatures of feeds 2', 3' and 4' are identical. The behavior of all stages is similar.

[0069] Only one master regulator, regulator OR25, is used, in combination with slave regulators OR26 to OR28.

[0070] The OR25 device and the OR26, OR27 and OR28 devices constitute a cascade regulation whose function is to control the CSP consumption of the process and to act on the temperatures of the feeds 2', 3' and 4' by means, respectively, of the temperature regulation devices H1, H2 and H3.

[0071] The desired setpoint value of the CSP0 consumption under nominal conditions is entered as SP_INT value of the OR25 device and the OR25 device receives as MEASUREMENT value the value of the CSP consumption calculated in real time. The output of the OR25 device is a working temperature setpoint sent in SP_EXT to each of the OR26, OR27 and OR28 devices.

[0072] The devices OR26, OR27 and OR28 compare the SP_EXT setpoint from OR25 with the temperature MEASUREMENT value of the power supply 2' of the first stage E1 measured by the temperature sensor T_F1, of the power supply 3' of the second stage E2 measured by the temperature sensor T_F2 and of the power supply 4' of the third stage E3 measured by the temperature sensor T_F3. The devices OR26, OR27 and OR28 then act on the temperature control devices H1, H2 and H3 respectively so that they modify the temperature of the currents 2, 3 and 4 respectively so that the power supplies 2', 3 / and 4' respectively are at the right temperature so that the consumption value CSP approaches the corresponding value CSP0.

[0073] The following Examples illustrate the present invention without, however, limiting its scope.

[0074] In these examples:

[0075] All percentages given are molar percentages.

[0076] The purification process used includes three membrane stages and follows the architecture of the process illustrated in the figure.

[0077] A raw gas composed, under nominal conditions, of 55% methane (component B), 44.3% carbon dioxide (component A), 0.2% oxygen and 0.5% nitrogen was separated. When the composition of the incoming raw gas is changed, the methane composition is varied and the carbon dioxide composition is adjusted to obtain 100%.

[0078] The raw gas flow rate is set at 400 Nm 3 / h under nominal conditions.

[0079] The temperature of the feed streams passing through the membranes is 25°C under nominal conditions. Pressure losses in the pipes and in the membranes are neglected.

[0080] The purification efficiency is defined as the molar flow rate of methane recovered from the retentate 5 divided by the molar flow rate of methane entering the raw gas 1. The value of the purification efficiency is set at 99.2%.

[0081] The supply pressure of stage E1 is set at 10 barg (1.0 MPa relative to the pressure gauge).

[0082] The permeate pressure of stage E2 is set at 0.1 barg (10000 Pa relative to the manometer).

[0083] The permeate pressure of stage E3 is set at 0.001 barg (100 Pa relative to the manometer).

[0084] The supply pressure of stage E3 is controlled by a control valve V1 located on the retentate of stage E3. This regulation aims to guarantee a purification efficiency of 99.2%.

[0085] Hollow fiber membranes marketed by UBE, type CO-810-FSC with selectivity 72.7 at 25°C, are used on the three separation stages, respectively 5, 4 and 5 membranes on stages E1, E2 and E3.

[0086] Specific consumption is defined as the quantity of electrical energy required by the process reduced to the quantity of raw gas treated. It is therefore expressed in kWhe / Nm 3 of raw gas.

[0087] In the Examples, it is calculated using the following formula (1)

[0088] (1)

[0089] Formula in which: Q is the raw gas flow rate in Nm3 / h; Pu COMP is the power required (in kW) by compressor C1 assuming an isentropic efficiency of 0.7;Pu GF is the power required (in kW) by the cooling unit located at the outlet of compressor C1 in order to cool the gas, assuming a coefficient of performance of 2.5;Pu AUXis the power required (in kW) by various auxiliaries necessary for the pretreatment of the raw gas before entering the purification process (not shown in the Figures).

[0090] Under nominal conditions, the following results are obtained: a process recycling rate of 0.2871. (Definition of recycling rate = sum of molar flow rates 6 and 7 divided by molar flow rate 1); a molar fraction of CH4 in retentate 5 of stage E2 of 97.34%; and a calculated theoretical electrical consumption of 0.190 kWhe / Nm 3 of raw gas.

[0091] These are the results that we seek to approach when the nominal flow rate of the raw gas or the nominal composition of the raw gas changes over time.

[0092] Example 1 (according to the invention) according to the diagram of:

[0093] The process diagram described in the.

[0094] Table 1 shows the process results after changing the incoming raw gas flow rate to 90%, 75% and 50% of the nominal flow rate value.

[0095]

[0096] As can be seen in Table 1, the regulation of RSC values i of the separation stages E1, E2 and E3 by adjusting the feed temperature of each stage allows the values ​​of the CH4 content in the retentate 5, the recycling rate and the electrical consumption to approach the values ​​obtained under nominal conditions while the values ​​obtained without regulation deviate much more. The electrical consumption increases in particular in the absence of regulation of the RSC values i because of the increase in the recycling rate when the feed rate decreases.

[0097] Example 2 (according to the invention) according to diagram of:

[0098] The process diagram described in the.

[0099] Table 2 shows the process results after changing the incoming raw gas flow rate to 50% and 150% of the nominal flow rate value.

[0100]

[0101] As can be seen in Table 2, the regulation of the TC values ​​on the methane of the separation stages E1, E2 and E3 by adjusting the feed temperature of each stage allows the values ​​of the CH4 content in the retentate 5, the recycling rate and the electrical consumption to approach the values ​​obtained under nominal conditions while the values ​​obtained without regulation deviate much further.

[0102] Example 3 (according to the invention) according to the diagram of:

[0103] The process diagram described in the.

[0104] Table 3 shows the process results after changing the incoming raw gas flow rate to 75%, 50%, 125% and 150% of the nominal flow rate value.

[0105]

[0106] As can be seen in Table 3, the regulation of the TCG values ​​on the methane of the separation stages E1, E2 and E3 by adjusting the feed temperature of each stage allows the values ​​of the CH4 content in the retentate 5, the recycling rate and the electrical consumption to approach the values ​​obtained under nominal conditions while the values ​​obtained without regulation deviate from them much more.

[0107] In this particular case where the membranes are identical on all stages and the controlled variable is the TCG, the supply temperature is the same for all three stages.

[0108] Table 4 shows the process results after changing the composition of the incoming raw gas to a methane mole fraction of 50% and 60%.

[0109]

[0110] As can be seen in Table 4, the regulation of the TCG values ​​on the methane of the separation stages E1, E2 and E3 by adjusting the feed temperature of each stage allows the values ​​of the CH4 content in the retentate 5, the recycling rate and the electrical consumption to approach the values ​​obtained under nominal conditions while the values ​​obtained without regulation deviate from them much more.

[0111] Example 4 (according to the invention) according to the diagram of:

[0112] The process diagram described in the.

[0113] Table 5 shows the process results after changing the incoming raw gas flow rate to 75%, 50%, 125% and 150% of the nominal flow rate value.

[0114]

[0115] As can be seen in Table 5, the regulation of CSP consumption by adjusting the supply temperature of each stage allows the values ​​of the CH4 content in the retentate 5, the recycling rate and the electricity consumption to approach the values ​​obtained under nominal conditions while the values ​​obtained without regulation deviate much further.

Claims

– Method for purifying a raw gas (1) by membrane separation, said method using n membrane separation stages, n being greater than or equal to 1, the or each membrane separation stage, denoted i with 1 ≤ i ≤ n, being constituted by:at least one separation membrane whose permeability depends on the temperature;a gas feed inlet of said stage i through which:for i=1, said raw gas to be treated (1) is introduced, where appropriate in combination when n=1 with a permeate recycling stream and / or a retentate recycling stream from the single stage and when n>1 with a permeate recycling stream (6) and / or a retentate recycling stream (7), said recycling streams coming from another stage; andfor i>1, the permeate or the retentate from one of the other stages is introduced,a retentate outlet;anda permeate outlet,said gas to be treated (1) consisting of a mixture of at least two constituents A and B, the permeability of the membranes for constituent A being greater than that of constituent B,characterized by the fact that it comprises the following steps:a nominal flow rate and / or a nominal composition of the incoming raw gas to be treated (1) is established, representing the nominal operating point;for each separation stage i, at least one nominal operating characteristic quantity chosen from:the RSC ratio; i 0, defined as the SC ratio A i 0 / SC B i 0 or SC B i 0 / SC A i 0,SC A i 0 and SC B i 0being each defined as the ratio of the partial flow rate of the constituent A or B respectively in the permeate stream leaving stage i, noted respectively Pe A i 0 and PeB i 0, on the partial flow of the same constituent in the supply of the same stage i, noted respectively F A i 0 and F B i 0; the charge rate of component A or B at stage i, noted respectively TC A i 0 and TC B i 0, defined as the ratio F A i 0 / P A i 0 or F B i 0 / P B i 0.F A i 0 and F B i 0 being the partial flow rate of component A or B in the supply of stage i and P A i 0 or P B i 0the permeability of the membranes at the temperature of the nominal operating point for component A or B on stage i;the overall loading rate of component A or B on stage i, noted respectively TCG A i 0 and TCG B i0, defined as the ratio of the flow rate of component A or B in the nominal flow rate to the permeability of the membranes at the temperature of the nominal operating point for the component A or B respectively on stage i, i.e. P A i 0 or P B i0; and in the case where one operates with several stages, where one uses identical membranes on all stages i and one establishes the same supply temperature on all stages i for the nominal flow rate and / or the nominal composition of the incoming raw gas to be treated, the specific consumption CSP0, defined as the energy consumption of the process divided by the nominal flow rate of an incoming raw gas to be treated (1) or of at least one stream leaving the process under the established nominal conditions;when the flow rate and / or composition of the incoming raw gas (1), measured before any possible mixing with at least one recycling stream, changes during the course of the process, the permeability of at least one membrane in at least one stage i is adjusted and thereby the quantity of constituents A and B passing through the membrane(s) concerned, by adjusting the temperature of the feed to the corresponding separation stage so that at any time, and independently for each stage i, at least one of the RSC ratio; i or the TC charge rate A i or TC B i or the overall TCG charge rate A i or TCG B i or the CSP, measured and / or calculated in real time on each separation stage, approaches its corresponding reference value, RSC i 0 or TC A i 0 or TC B i 0 or TCG A i 0 or TCG B i0 or CSP0. – Method according to claim 1, in which one operates with several stages, characterized in that identical membranes are used on all stages i and the same feed temperature is established on all stages i for the nominal flow rate and / or the nominal composition of the raw gas entering to be treated, the overall load rate TCG is chosen as the nominal characteristic quantity A i 0 or TCG B i 0, and in the case of a change in the flow rate and / or composition of the incoming raw gas, the feed temperature is adjusted in all stages using a single regulator associated with the overall load rate at any stage. – Method according to one of claims 1 and 2, characterized in that at any time and in real time and for each stage i, the permeability of the membranes is determined for the constituent A or B, i.e. P A i (t) or P B i(t) by measurement and / or calculation of the pressures, temperatures, flow rates and compositions of the incoming and outgoing streams, and by numerical resolution. - Method according to one of claims 1 to 3, characterized in that the separation membranes are polymer membranes chosen from polysulfone membranes and polyimide membranes. – Method according to one of claims 1 to 4, characterized in that the temperature of the supply of each stage is adjusted using a heat exchanger. - Method according to one of claims 1 to 5, characterized in that the constituent A of the raw gas is carbon dioxide, and the constituent B is methane.

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